Ocean-going vessels (e.g. oil tankers, bulk carriers, container carriers and offshore support vessels) play an indispensible role in the globalised world for transporting freight between markets and for supporting offshore oil and gas exploration and production activities in energy sector. From the global growth of marine industry, it is projected that ocean-going vessels will become a significant source of pollution in the near future. These include oily sludge, hazardous substances, wastewater, garbage, atmospheric emissions and ballast water. Presently, challenges encountering the marine industry include the implementation of upcoming regulations, resulting in the pressure, particularly, to reduce the atmospheric pollutants and to treat ballast water produced from ships.
At the international level, atmospheric emissions from ships are regulated under Annex VI of the International Convention for the Prevention of Pollution from Ship (MARPOL 73/78) set by International Maritime Organization (IMO). Annex VI applies to all ships of 400 gross tonnages (GT) and above and to fixed and floating drilling rigs and other platforms with exemption of emissions from sea-bed mining activities, i.e. exploration, exploitation and associated offshore processing of sea-bed mineral resources. Provision of Annex VI covers three major gaseous pollutants produced from diesel: engines, i.e. SOx, NOx and PM and the standards are divided into two sets, i.e. global requirement and more stringent requirement in Emission Control Area (ECA).
The emission of SOx is regulated through the content of sulfur in fuel oil used onboard ships; The most stringent regulation on SOx and PM appears to be in Year 2015 (Sulfur<0.1% w/w) for ECA and in Year 2020 (Sulfur<0.5% w/w) globally. To comply with the requirement, ship owners must either switch to fuel with low sulfur content or instigate cleaning of the ship's exhaust gases. Due to the potential fluctuation of low sulfur fuel cost, the latter option is most likely to be more viable.
SOx Removal
A process for SOx removal from flue gas, referred to as desulfurization, can be classified into two techniques, i.e. wet and dry scrubbing methods.
Wet scrubbers are usually applied for SOx removal from flue gas with the utilization of basic scrubbing agents (i.e. seawater or alkaline substances dissolved in freshwater). The SOx containing exhaust gas is brought into a scrubber in full contact with the scrubbing agents; it is captured by neutralization mechanism and converted to sulphite or bisulphite forms in the scrubbing liquid phase. The spent seawater after absorption is then aerated and further neutralized so that it can be returned to the sea without damaging to the marine environment. Since this technology relies only on the alkalinity of seawater obtained and the amount of scrubbing water is reversely proportional to seawater alkalinity, the variation of seawater conditions will affect tremendously the removal efficiency of SO2 from exhaust gas. For example in Baltic sea ECA, the seawater alkalinity varies from 500 μmol/kg to 2200 μmol/kg, which means that a seawater scrubber must be able to take in four times more seawater flow if it is designed at normal seawater alkalinity (2200˜2400 μmol/kg) level, which is extremely difficult for practical implementation. Furthermore, high amount of seawater flow to the scrubber induces significant back pressure to the exhaust line, creating a high risk of stalling the main engine.
Based on the above consideration, it is obvious that reducing the scrubbing sea water flow rate is critical. Unfortunately, at natural seawater conditions (where there is no dilution from river and rainfall) the alkalinity of seawater is too low to achieve satisfactory SO2 removal without incurring intensive energy input to drive the pumps for scrubbing systems. For example, a marine engine fuelled by HFO with 3.5 wt % sulphur, every 1 MWh of brake power produced requires at least 171 m3 of seawater to capture and neutralize the SO2 from exhaust gas. The huge amounts of seawater has to be pumped, contacted with flue gas and then treated after scrubbing, and as a result these processes require large and costly equipment when installed onboard a ship where space limitation and retrofitting are major concerns.
NOx Removal
The limit of NOx emissions from ships is regulated under Annex VI and applies to the emission from diesel engines with a power output of greater than 130 kW. The most stringent standard is applied to ships with a keel-laying date on or after 1 Jan. 2016 operating in ECA. This standard represents around 80% less of NOx reduction from the NOx emission from most engines currently used worldwide.
To comply with the requirement, two major techniques can be applied and they are namely engine modification and post-combustion treatment. It is unlikely that the engine modification can be a stand-alone technique to facilitate the NOx reduction to meet the IMO requirement. For post-combustion treatment, selective catalytic reduction (SCR) seems to be the only dry method currently employed to assist the ship owners to meet the IMO requirement. However, there are several drawbacks of SCR and these typically include catalyst poisoning by the presence of SO2 in flue gas, its complicated operation and ultimately its cost. To address these issues, a robust and self regenerative process for onboard NOx removal is desirable.
Ballast Water Management
The transport of ballast water containing aquatic organisms can also cause a serious environmental threat to the world's oceans and sea. The spreading of invasive species results in irreversible damage to biodiversity and the valuable natural resources.
Under the International. Convention for the Control and Management of Ships'Ballast Water and Sediments, IMO regulations require all newly built vessels to comply with ballast water treatment standards from Year 2009 or Year 2011 depending on ship size. By Year 2016, compliance with these new standards will be compulsory for all vessels.
In general, the ballast water treatment can be divided into two steps, i.e. solid-liquid separation and disinfection. Solid-liquid separation is utilized for the separation of solid material, including large suspended microorganisms from ballast water prior to undergoing disinfection.
Disinfection removes and/or deactivates microorganisms. There are various disinfection techniques, i.e. physicochemical disinfection (e.g. ultraviolet light) and chemical disinfection (e.g. ozonation, chlorination and electrochlorination). The application of ultraviolet is limited by the turbidity of ballast water. Chemical disinfection is an attractive technique in comparison with physicochemical disinfection.
US 2004/0099608A1 (Leffler et al.), US 2004/6773611B2 (Perlich et al.), US 2007/7244348 B2 (Fernandez et al.) disclose systems and apparatuses for ballast water treatment to remove contaminants from ballast water on a vessel using disinfectants generated from electrolysis of saltwater. The ballast water may be disposed overboard after being treated.
The disinfectant include chlorine (Cl2), bromine (Br2) and other halides, hypochlorite (ClO−), chlorine dioxide (ClO2), hydrogen peroxide (H2O2) or other disinfectants into ballast water. Chlorine is a primary disinfectant that has been widely used in water treatment industry. Chlorine can be generated from electrolysis of saltwater and seawater. At the same time, alkaline substances, including sodium hydroxide (NaOH) is also generated as another product.
PAJ 52-128886 (1977) discloses a method to prevent the sticking of microorganism in seawater, by equipping the seawater electrolyzer in the ship and carrying out desulfurization by supplying NaOH gained from the above electrolyzer, to the scrubber cooling water, together with supplying Cl2 generated from the above equalizer and from the electrolyzer, to the piping of seawater system.
According to above said demand on marine emission control (SO2 and NOx removal) and ballast water treatment technology, improvements to overcome the drawbacks of current technologies while complying with forthcoming regulations on emissions and ballast water are highly desirable.